Plastic optical fibers (POF) which comprise both a core and cladding of plastic material, i.e., polymers, which may or may not be combined with other materials have gained increased usage for short haul signal transmission lines. Generally speaking, such fibers are considerably less costly than typical glass optical fiber. However, they have higher losses than the latter, hence use is restricted to short distances, such as LAN (local area network) and ISDN (integrated service digital network) where they have proven to be quite useful. In general, such a POF may comprise a core of, for example, an optical polymer of polymethyl methacrylate (PMMA) and a polymeric cladding therefor. In general, a plastic optical fiber is encased in a jacket layer for protection of the enclosed fiber. In many types of POF, the jacket layer is applied after the fiber is drawn from the perform. This jacket layer may be either strippable or non-strippable. A strippable jacket typically comprises a material with very weak adhesion to the cladding material, and is stripped away at a fiber end during connector attachment. A non-strippable jacket typically comprises a material with very strong adhesion to the cladding material, and connectors are typically applied over such a jacket. In the latter case, the jacket material is often chemically very similar to the cladding material, and may even be chemically identical.
Unlike silica based optical fiber (glass) wherein the fiber is drawn from a preform in a draw furnace where heat transfer is achieved primarily by radiation, POF is most often drawn from a resistive furnace wherein heat transfer occurs largely by conduction, mainly because much lower temperatures (200°-300° C.) are involved. At such temperatures, the blackbody radiation from a typical electrical resistive furnace occurs predominantly at mid-infrared wavelengths of ten microns (μm) or longer, which, unfortunately is a region where optical absorption of the polymers is extremely strong. In such a situation, heat transfer into the central region of the preform is very slow and is accompanied by unacceptable temperature gradients across the preform. This restriction on heat transfer ultimately limits the maximum draw speed as well as the preform size that may be used in POF production, thereby leading to high production costs and reducing the advantage of POF over glass fiber.
The heat transfer process has been improved in the prior art through the use of infrared lamp furnaces, which are typically constructed from arrays of halogen lamps. Such lamps usually have filaments that operate at temperatures approaching 2500K and thus emit radiation at wavelengths that are predominantly in the one to two micron (1-2 μm) range. In this wavelength range, typical optical polymers such as are used in POF, such as polymethyl methacrylate, are considerably more transparent than at the longer wavelengths. As a consequence, the incident radiation from the lamps penetrates farther into the preform, with a consequent more uniform and more rapid heat transfer. Despite this marked improvement, even in the 1-2 μm band, optical polymers such as PMMA absorb a large fraction of the incident radiation within a thickness of fifteen to twenty millimeters (15-20 mm). Thus, even with infrared lamp heating, heat transfer processes still place significant limitations on preform diameter and draw speed.